build: drop the isatty gnulib module

The isatty gnulib module adds a fix for Win32 platforms where it doesn't
work correctly with character devices like NUL. This is not a compelling
enough problem for libvirt to be concerned with.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

bootstrap.conf

@@ -52,7 +52,6 @@ gettimeofday
 gnumakefile
 intprops
 ioctl
-isatty
 largefile
 listen
 localeconv